1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
An apparatus using an array of individually controllable elements may project radiation onto a substrate using an arrangement known as “pixel grid imaging.” In such an arrangement, a micro lens array is used to focus the radiation from each of the individually controllable elements within the array of individually controllable elements onto a spot on the substrate. As radiation is projected onto the substrate, the substrate is scanned at a constant velocity. Therefore, while radiation is directed onto a lens in the micro lens array, a strip of radiation is projected onto the substrate.
The length and position of the strip in the direction of scanning of the substrate is controlled by the array of individually controllable elements, which determines when radiation is directed to the lens and when it is not. The system is configured such that different spots associated with different lenses in the micro lens array can project strips of radiation onto the substrate that are adjacent to each other. Accordingly, a patterned feature can be built up by a plurality of such adjacent strips of the appropriate length and in the appropriate positions.
However, the illumination source is imaged at the substrate. Consequently, the spot projected on the substrate by each lens of the micro-lens array is dependent on the illumination source shape. In turn, this means that the quality of the image generated is dependent on the illumination source shape. Furthermore, the effective source shape may vary over the image field. The effective illumination source shape may vary, for example, due to aberrations in the optics or due to speckle. However, it is desirable for the illumination across the array of individually controllable elements to be uniform and without variations such as speckle.
Therefore, what is needed is a system and method that reduces the variations in image quality caused by variations in an illumination source shape.